I'm writing a rapport about my simulator and I'm writing the chapter about jPCT structure... in your notes you draw, using a solid line, that an Object3D has to have a texture.... but, I've used objs in my applet in which I've applied just simple material without textures, so I'm not pretty sure to understand this necessary relation.....

That line is a requirement caused by the actual implementation, it's not a conceptual one. In jPCT every object has at least one texture, even if you haven't assigned one. This can either be a default, plain white dummy texture that the texture manager provides or a colored texture that matches the color of your material. This is (as said) implementation specific and it could change without any consequences for the user of the API (even if it most likely won't...), so i think you are save to write that an object may have a texture...but doesn't have to.